Well, there was a lot of empty space to go around. The earliest migrants seem to have ended up in South America, so they probably got pushed there by the later migrants.
True- I wonder though what happened when the later Indians came into contact with the previous dwellers? Did,they fight and take their land away? We know that later when I dians were abundant, that tribes always fought and took land from other Indians, (not all, some were peaceful of course)- but when white folks did the same, we were the ones Called violent occupiers.